Spartobranchus tenuis lived about 505 million years ago, and its fossils are commonly found in the Burgess Shale fossil beds of Yoho National Park. (Jean-Bertrand Caron/Royal Ontario Museum)

While prehistoric animals of diverse shapes and sizes arose and then went extinct over hundreds of millions of years, one beach-dwelling sea creature has stubbornly stayed the same — maintaining almost exactly the same phallus-shaped form from the time of the trilobites through the rise and fall of the dinosaurs to the present day.

Canadian and British researchers have finally identified a type of common fossil from the Burgess Shale fossil beds of B.C.'s Yoho National Park that had puzzled paleontologists for a century.

It turns out that the fossils belonged to animals that lived 505 million years ago, but that are almost identical to acorn worms, which still live on seashores around the world, including Canada's East, West and Arctic coasts.

That makes the fossil animal, named Spartobranchus tenuis, unusual among the Burgess Shale fossils. Many of those represent animals whose descendants have either long been extinct or have undergone 500 million years of drastic change through the process of evolution.

"We often look at the Burgess fossils as odd and bizarre … there actually are some very strange creatures in the Burgess Shale," said Jean-Bernard Caron, a paleontologist at the Royal Ontario Museum in Toronto, in an interview.

"But this one [Spartobranchus] is actually remarkably similar to modern form."

Caron led the research identifying the animal, which was published Wednesday online in the journal Nature.

Caron said fossils of the animal were first collected by Charles Walcott, the American paleontologist who discovered the Burgess Shale in 1909. But they were never described or identified.

Thousands of specimens — 3,000 at the Royal Ontario Museum alone — sat in drawers at museums for decades, awaiting study.

Spartobranchus looked very similar to a modern acorn worm such as the one above, which burrow along seashores around the world. (Chris Cameron/University of Montreal)

A few years ago, Caron became interested in the fossils and sifted through thousands of specimens to find the very few that were well preserved and suitable for study. He spoke to paleontologist Simon Conway Morris at the University of Cambridge, who suggested the fossils might belong to a group of animals called "enteropneusts," which include the acorn worms.

Instant recognition

Caron contacted Chris Cameron at the University of Montreal, who studies modern-day acorn worms. He has discovered 20 new species in North America in recent years, most recently the first Arctic species.

The fossil appears to represent a missing link between acorn worms and pterobranchs such as the one above, which has tentacles, lives in colonies, and builds a tube to live in. (Chris Cameron/University of Montreal)

Cameron recognized the fossil animal right away.

"Spartobranchus is clearly an acorn worm," he said in an interview. "It's almost like someone took a picture of a modern-day animal — it's absolutely astonishing."

Acorn worms aren't related to earthworms or parasitic worms such as roundworms. Instead, they belong to a group of animals called hemichordates that are more closely related to sea urchins and starfish; and to chordates, the group of animals that includes humans and many other animals with a backbone.

Acorn worms are typically about the size of an earthworm and burrow in the sand of mudflats along the sea shore, Cameron said. Many of them feed by pumping water into their gills to filter food particles.

Spartobranchus's very similar features suggested that it, too, fed that way.

Losing a tube

But there was one major difference between Spartobranchus and modern acorn worms.

One of the bizarre features that Caron noticed when he stumbled across the worms was that they were found inside tubular structures of various shapes — some that were straight tubes, some that were branched, and some that were ring shaped. "I thought, 'Whoa, what is that? Looks like small towers on the rock,'" he recalled.

It turns out that another type of hemichordate called a pterobranch — a tentacled, deep sea creature that lives in colonies and looks nothing like an acorn worm — builds and lives in similar fibrous tubes. DNA evidence had suggested that pterobranchs and acorn worms were related, but no one could figure out how.

It now appears that Spartobranchus may have been a common ancestor to both acorn worms and pterobranchs, or some kind of transitory form at the time when the two types of animals diverged from a common ancestor. In that sense, it fills in a missing link between the two groups of hemichordates.

"That's really the power of fossils that you can find these things that look like intermediates that really bring to life what might have happened so many millions of years ago," Cameron said.